Photo-emissive electron device



May 2, 1967 P. D. WILLIAMS PHOTOEMISSIVE ELECTRON DEVICE Filed Oct. 31.1963 INVENTOR. PAUL D. WILLIAMS Wig/ ad, FM 7 ATTORNEYS United StatesEatent ()fi 3,317,? Patented May 2, 1957 3,317,771 PHOTO-EMISSIVEELECTRON DEVICE Paul D. Williams, Portola Valley, Califi, assignor, bymesne assignments, to Varian Associates, a corporation of CaliforniaFiled Oct. 31, 1963, Ser. No. 320,314 Claims. (Cl. 31394) This inventionrelates to electron emissive devices and more particularly to suchdevices in Which electron emission is caused by light striking aphoto-emissive surface.

The main object of this invention is to provide improved structures formaking use of the well known phenomenon of photo emission.

A more specific object of the invention is to provide an electron tubein which the cathode is a photo emitter and in which the interelectrodespacing is closer than has heretofore been possible. The maximumfrequency at which an electron tube can operate is limited by thespacing between electrodes; the closer the interelectrode spacing, thehigher the frequency. In conventional electron tubes employing thermallyemissive cathodes, the high operating temperature limits the closenessof electrode spacing because of thermal distortion of parts. Inconventional photo emissive tubes, substantial electrode spacing isrequired in order to provide a path for light to reach the photoemissive cathode surface. Modern requirements for electron tubeoperation at increasingly higher frequencies makes it necessary to finda solution to the problem of close electrode spacings.

Another and releated object of the invention is to provide photoemissive tubes in which it is possible for given size electrodes to beenclosed in a smaller tube envelope than was heretofore possible withconventional photo emissive tubes. In contrast to the inventionconstruction, conventional photo emissive tubes require relatively largelight-admitting envelopes to contain given size electrodes. Since photoemissive tubes are light-operated, the modern advances in the field oflight transmission, such as the development of lasers, makes itincreasingly necessary to provide improved light-operated tubes.

An additional object of the invention is to provide a photo emissivetube in which the activating light can pass through two opposite Wallsof the tube envelope.

An additional object of the invention is to provide an improved methodfor making close spaced electrodes particularly for photo emissiveelectron tubes.

By way of brief description, the objects of the invention are achievedaccording to one embodiment by a photo emissive structure in which theanode is made of transparent material, such as glass or quartz, having atransparent electrically conductive surface, the structure being soarranged that the light which activates the cathode passes through theanode. According to another embodiment of the invention, the cathode ismade of transparent material through which the activating light passes.In a further embodiment, both the anode and cathode are transparent. Thedescribed construction makes it possible for the electrodes to beextremely close spaced. In other words, light-admitting space is notrequired between the electrodes. Instead the light passes through one ormore of the end electrodes. In some embodiments one or more of thetransparent electrodes serve as walls of the tube envelope to decreasethe overall envelope size.

These and other object and features of advantage will be apparent tothose skilled in the art from the following detailed description whereinreference is made to the accompanying drawings in which:

FIGURE 1 is a cross-sectional view on the center line of an electronemissive device according to the invention in the form of an extremelyclose spaced diode;

FIGURE 2 is a cross-sectional view on reduced scale showing the anode ofFIGURE 1 being processed in a bell jar to form spacers between the anodeand cathode;

FIGURE 3 is an exploded perspective view of the anode and mask fromFIGURE 2 on a larger scale;

FIGURE 4 is a perspective view on enlarged scale of the anode afterbeing processed as in the apparatus of FIGURE 2.

Referring to the drawings in more detail, FIGURE 1 shows an extremelyclose spaced diode 10. The diode comprises a photo emissive cathode 11and a transparent electron receiving electrode or anode 12. The anodecomprises a quartz or glass disk 13, preferably having its lower surface14 and its upper surface 15 made optically flat. The upper surface 15 iscoated with a transparent electrically conductive coating 16 whichextends around the periphery of disk 13 and forms an annular terminalring on the bottom surface 14. Transparent conductive coating processesare Well known in the art, and any such suitable process can be employedto apply coating 16; for example, the process disclosed in BritishPatent 632,256, dated Nov. 18, 1949.

The cathode 11 is preferably formed in identical manner as described forthe anode 12, including an optically fiat quartz or glass disk 26 and anelectrically conductive transparent coating 21. In addition, the cathodehas a conventional photo emissive coating 22 on the conductive coating21. The spacing between the cathode and anode is provided by a pluralityof dielectric spacer disks 23 as will be hereinafter described in moredetail. The thickness of coatings 16, 21, 22 and 23 are shown many timesgreater than actual in order to be visible.

The cathode and anode are positioned in an envelope comprising asemi-cylindrical glass dome 25 which allows light from any suitablesource to enter the envelope. A metal anode terminal ring 26 is bondedon one side to dome 25 and on the other side to a cylindrical glass sidewall 27. The upper end of the envelope is formed by a metal sealing ring28 bonded to wall 27, and a metal closure disk 29 having an upturned rimreceived Within the sealing ring 28. The members 28 and 29 are brazed orheliarc welded together as at 30. The members 28 and 29 serve as anexternal terminal for the cathode. The interior of the envelope isevacuated, either by means of conventional pinch-off tabulation (notshown) or by making the final seal in a vacuum chamber in which the tubewas evacuated prior to making the seal.

The cathode and anode are held in place by metal coil springs 31 and 32.Spring 31 abuts the ring 26 and the annular portions of the conductivecoating 16 on the bottom of the anode. Spring 32 abuts the closure disk29 and the annular portion of the conductive coating 21 on the top ofthe cathode. Thus, the springs 31 and 32 serve as electrical leads forthe anode and cathode, and also serve to position the anode and cathodein the envelope. The springs also serve to position the anode andcathode relative to each other by constantly forcing the anode andcathode toward each other so that the spacing therebetween is alwaysexactly the thickness of the spacers 23. Assembly of the device can beeasily accomplished by first preparing parts 25, 26 and 27 as asubassembly and then inserting parts 31, 12, 11, 32 and 29 in the ordernamed.

Formation of the spacer dis-ks 23 is shown in FIGURES 24. FIGURE 2 showsa conventional glass bell jar 25 removably sealed on a metal base plate36. The base plate is apertured to receive an exhaustv pipe 38 connectedto a pump (not shown) for the purpose of evacuating the bell jar. A pairof metal support posts and conductors 39 are mounted in the base plateand insulated therefrom by dielectric sleeves 40. Each of the posts isbored to receive the ends of a heating wire 41, such as tungsten, and athumb screw 42 in each post locks the Wire in place.

Wire 41 is coated, at least along its central portion, with avaporizabie dielectric material 43, such as silicon dioxide.

The anode 12 is positioned on the base plate with the coated surface 16upward. Then the anode is covered with a thin masking plate 46preferably of metal and preferably having a downwardly extending rim 47for automatically centering the plate on the anode. FIGURE 3 is anenlarged perspective view of plate 46 about to be lowered onto the anodewhich has been previously coated with the transparent conductive layer16. The masking plate has at least three holes 48, or other suitablyshaped type of aperture, through which vaporized silicon dioxide canpass to form spacers which will hold the anode and cathode disks apartwithout permitting the disks to tilt.

Deposition of silicon dioxide 43 is performed by first evacuating belljar 35 and then passing current through wire 41 via posts 39 until thewire is hot enough to vaporize the silicon dioxide. The vaporizedsilicon dioxide passes through holes 48 in the masking plate andcondenses on the anode. FIGURE 4 shows anode 12 after spacers 23 havebeen deposited by the described process.

The thickness of the deposition can be very accurately controlled byadjusting such variables as amount of silicon dioxide on wire 41, timeof heating current, and distance between the anode and coating 43.Because vapor deposition deposits particles of atomic or molecular size,the thickness of spacers 12 can be made extremely small with greataccuracy and reproducibility. For example, the spacers 23 can be made tohave a thickness of one micron or even substantially less. Since thespacing between the anode and cathode is the same as the thickness ofspacers 23, unprecedented minimum electrode spacing can be obtained. Thespacing can even be made so close that tube need not be evacuatedbecause the length of electron travel will be so short that thestatistical likelihood of elec trons striking air molecules will nolonger be a problem. The interelectrode spacing will not changeperceptibly during operation for two reasons. One reason is that a photoemissive cathode operates at relatively low temperature compared to aconventional thermally emissive cathode. The other reason is that evenif the light source or ambient thermal conditions do create substantialheat, silicon dioxide has an extremely low coeflicient of expansion, andbecause of the spring arrangements 31 and 32, the spacing is determinedsolely by the silicon dioxide spacers 23.

In order to operate the device of FIGURE 1, any conventional lightsource (not shown) is positioned outside the device so that the lighttherefrom will enter through the glass dome 25. The light passes throughthe transparent anode 13, through the transparent conductive coating 16,and strikes the photo emissive surface 22. The photo emissive surfacereacts to the light energy and emits electrons which travel to theconductive surface 16, thus completing the circuit through the device.The electron emissive device 10 can be used in the same manner as aconventional photo emissive tube, and is particularly suitable for useas receiver for information transmitted by a laser in the form of a beamof light.

Although the specific arrangement described for FIG- URE l is preferred,the dielectric spacers 23 can be deposited on the cathode 11 instead ofon the anode 12. Similarly, it should be understood that the cathode andanode unit can be inverted from the position in FIGURE 1. In theinverted arrangement the light does not reach the photo emissive surface22. by passing through the transparent anode. Instead the light reachesthe photo emissive surface by passing through the transparent cathode.Regardless of whether the cathode and anode are arranged as shown inFIGURE 1 or inverted, it should 4. be understood that the upperelectrode in FIGURE 1 need not be made transparent and can be made ofmetal. If the upper electrode is made of metal, it will be coateddirectly with the photoemissive material 22 when used as the cathode andneed not be coated at all when used as the anode.

Although preferred embodiments of the present invention are shown anddescribed herein, it is to be understood that modifications may be madetherein without departing from the spirit and scope of the invention asset forth in the appended claims.

What is claimed is:

1. An electron discharge device comprising an envelope having acylindrical dielectric side wall, an electrically conductive closuremember attached to one end of said cylinder, an electrically conductiveterminal ring attached to the other end of said cylinder, a transparentclosure member attached to said terminal ring, a transparent anode diskin said cylinder, said disk having a transparent electrically conductivecoating on its surface facing away from said transparent closure member,means forming an electrical connection between said transparentconductive coating and said terminal ring, a cathode disk between saidanode disk and said conductive closure member, said cathode disk havinga photoemissive surface facing said anode, dielectric spacing materialbetween and abutting said disks, and means forming an electricalconnection between said photo emissive surface and said closure member.

2. An electron discharge device as claimed in claim 1 in which saidmeans forming an electrical connection for said anode comprises a coilspring between said anode disk and said terminal ring.

3. An electron discharge device as claimed in claim 1 in which saidmeans forming an electrical connection for said cathode comprises a coilspring between said cathode disk and said conductive closure member.

4. An electron discharge device as claimed in claim 3 in which saidmeans forming an electrical connection for said anode comprises a coilspring between said anode disk and said terminal ring.

5. An electron discharge device comprising an anode disk and a cathodedisk having major surfaces with electron active portions in close spacedequidistant juxtaposition, the spacing between said electron activeportions being of the order of microns, dielectric spacing materialbetween and abutting said major surfaces of said disks at points removedfrom said electron active portions, said major surface of said cathodedisk facing said anode disk having a photo emissive coating thereon,said major surface of said anode disk facing said cathode disk beingconductive, one of said disks being transparent, means forming anelectrical connection to said photo emissive coating on said cathodedisk, and means forming an electrical connection to said conductivesurface of said anode disk.

References Cited by the Examiner UNITED STATES PATENTS 1,965,849 7/1934McIlvaine 313--102 X 2,451,400 10/ 1948 McIlvaine 3 l3-102 X 2,490,74012/1949 Nicoll 313-94 X 2,876,374 3/1959 Riggen 313102 2,894,167 7/1959Day 313102 X 3,093,507 6/1963 Lander et al 117-212 X 3,116,427 12/1963Giaever 30788.5 3,214,629 10/1965 Apker 313346 JAMES W. LAWRENCE,Primary Examiner.

P. C. DEMEO, Assistant Examiner.

1. AN ELECTRON DISCHARGE DEVICE COMPRISING AN ENVELOPE HAVING ACYLINDRICAL DIELECTRIC SIDE WALL, AN ELECTRICALLY CONDUCTIVE CLOSUREMEMBER ATTACHED TO ONE END OF SAID CYLINDER, AN ELECTRICALLY CONDUCTIVETERMINAL RING ATTACHED TO THE OTHER END OF SAID CYLINDER, A TRANSPARENTCLOSURE MEMBER ATTACHED TO SAID TERMINAL RING, A TRANSPARENA ANODE DISKIN SAID CYLINDER, SAID DISK HAVING A TRANSPARENT ELECTRICALLY CONDUCTIVECOATING ON ITS SURFACE FACING AWAY FROM SAID TRANSPARENT CLOSURE MEMBER,MEANS FORMING AN ELECTRICAL CONNECTION BETWEEN SAID TRANSPARENTCONDUCTIVE COATING AND SAID TERMINAL RING, A CATHODE DISK BETWEEN SAIDANODE DISK AND SAID CONDUCTIVE CLOSURE MEMBER, SAID CATHODE DISK HAVINGA PHOTOEMISSIVE SURFACE FACING SAID ANODE, DIELECTRIC SPACING MATERIALBETWEEN AND ABUTTING SAID DISKS, AND MEANS FORMING AN ELECTRICALCONNECTION BETWEEN SAID PHOTO EMISSIVE SURFACE AND SAID CLOSURE MEMBER.